1. Field of the Invention
The invention relates generally to pressure laminated wood products and methods of making the same. More specifically, the invention relates to low-pressure laminated wood products and a method of securing decorative veneers to core substrates using resin-saturated sheets.
2. Brief Description of Related Technology
Layered, composite wood structures having an inner core substrate and veneer layers affixed to one or more major planer surfaces of the inner core substrate are well-known in the art. Such composite wood structures exhibit the desirable characteristics of the veneer material while providing a cost savings over a solid structure made entirely of the veneer material. For example, a natural hardwood veneer, such as oak, may be secured to a less expensive substrate, such as high density fiberboard, to produce a composite panel article. The composite wood structures serve a variety of uses, such as floorboards, doors, cabinets, countertops, and walls, to name a few. Veneers used in forming the composite wood structures are selected from a wide variety of wood materials depending upon economic considerations and the end-use of the structure. Typical woods used as veneers in the past have included ash, birch, cherry, maple, oak, pine, poplar, and walnut.
Wood veneers may be secured to core substrates by lamination processes using resin binders in the form of resin-coated papers. Commonly used resins include thermosetting materials, such as melamine resins. Melamine resins, also known in the art as melamine/formaldehyde resins, typically exist in aqueous solution, although alcohols or other organics may also be used as solvents. A melamine-coated paper can be dried to remove water and/or solvent leaving a dry, solid state melamine-coated paper.
As described in Kirk-Othmer's Encyclopedia of Chemical Technology, Volume 14, pgs. 1074-91 (1995), a resin-coated paper may be placed between a veneer sheet and an inner core substrate to form a layered, composite wood structure. This layered structure is then heated in a compression molding press at a temperature of 130.degree. C. (266.degree. F.) to 150.degree. C. (302.degree. F.) for 20 minutes to 40 minutes. The initial heating causes the resin to melt and flow into the voids and pores of the core substrate and the veneer sheet. With a sufficiently long press cycle, the resin polymerizes and crosslinks into an intractable network structure (a thermoset) bonding the core substrate to the veneer. After heating, the structure is cooled while in the press under pressure, and thereafter may undergo finishing operations, such as trimming and/or sanding, to form the final product.
Thermosetting materials, such as melamine, have been used due to their superior resistance to discoloration when subjected to the conventional lamination temperatures and their exceptional bonding strength. Layered wood composite structures made by a conventional compression molding press processes, as described above, however, may experience excessive warpage toward the veneer side.
Wood veneer and substrate materials generally contain moisture. Changes in moisture content as a result of changes in atmospheric moisture conditions and wetting of the wood surface(s) by water generally tend to cause wood materials to warp or blister. The type of manufacturing process also may influence the moisture content of the formed wood product and thereby affect warpage. Thus, the original moisture content of the veneer and substrate, as well as the amount of water added by the aforementioned binder, must be maintained low to avoid warp and/or blistering of the wood materials. During heat curing and/or pressing of the wood materials, water evaporates from the wood materials. This evaporation may impart stresses in the wood materials, causing them to warp. The evaporation of additional moisture from the binder used to secure the various wood materials together may also contribute to wood warpage.
Balancing veneer or heavy duty backer paper have been used in the past to balance, or offset, the wood warping problem. In this case, where a veneer layer is placed on both faces of a substrate, any tendency of the wood materials to warp effectively cancel each other out. However, the use of this additional veneer as well as board production is unnecessarily expensive where a second veneer serves no cosmetic purpose. Furthermore, a second veneer is often not desired in certain applications, such as store display boxes (e.g., jewelery display cabinets).
Thus, it would be desirable to have a cost-efficient process for making a laminated wood structure wherein the amount of warpage is drastically reduced, if not eliminated altogether.